I don't think you would notice much gas mileage difference driving the lower speeds you mention. If a lot of freeway driving, may make 1-2 mpg difference, just guessing.
I think the 4.30s are right for towing the FW weight you mentioned. Would the 3.73s work? Sure, but I think you'll be happier with the 4.30s, after the wait is over.
Another thing, is tire/wheel size. I purposely stayed with the stock 245/17s, when I ordered, wanting a lower bed height, but also to maintain the actual 3.73 ratio. The larger tires/wheels, will make the ratio higher, no matter the gear set.
You will like your new truck!
Jerry